The present invention relates to medical imaging systems in general, and in particular to Doppler untrasound imaging systems.
Ultrasound imaging is a commonly used technique for non-invasively imaging a patient""s internal tissue and organs as well as for analyzing which ultrasonic sound waves travel in the body. The maximum depth of tissue that can be analyzed with Doppler pulses is limited to one half of the distance that a Doppler pulse can travel between successive pulses. For example, at a PRF of 40 KHz, the maximum depth of tissue that can be analyzed by Doppler pulses that travel at 1.54 mm/xcexcsec is 19.25 mm. However, to accurately analyze blood flow within the heart, it is often necessary to analyze blood flow at depths of 80 to 100 mm.
Another approach used in conventional pulsed Doppler systems is called multi-pulse Doppler, where Doppler pulses are periodically transmitted and echo signals are received between transmission time. This approach products high sample rate echo signals at greater tissue depths. However, the range gate size is limited because of the time required to transmit the Doppler pulses. For example, the 19.25 mm range gate size for Doppler pulses transmitted at 40 KHz is often limited to 5 mm in practice due to the delays inherent in transmitting the pulses.
To increase the amount of echo data that can be used to analyze blood flow, many ultrasound systems transmit continuous wave (CW) Doppler signals into the body. Analog Doppler systems do allow high sample rates, good depths and large range gate sizes. However, such systems generally require a separate analog beamformer to receive the corresponding echo signals, which increases the cost and complexity of the ultrasound system. Digital CW beamformers have not generally been successful because the difference in magnitude between the Doppler pulses transmitted into the patient and the relatively weak echo signals received is too great for the digital beamformer to interpret accurately.
Given these problems associated with Doppler ultrasound imaging, there is a need for a method of increasing the amount of echo data that can be used to analyze blood flow without the use of additional hardware or sacrificing accuracy.
To address the limitations discussed above, the present invention is a method for obtaining Doppler echo data at relatively deep positions in the body, at high sample rates, and with a large range gate size without an analog beamformer.
According to a first embodiment of the invention, a Doppler pulse is transmitted for a time equal to the time required for an ultrasonic sound wave to travel from an ultrasound transducer to a point in the tissue defined by a top of the range gate and back. Following the transmission of this long Doppler pulse, the ultrasound system receives echo signals for a time equal to the time required for an ultrasonic sound wave to travel from the ultrasound transducer to the top of the range and back plus the time required for the ultrasonic sound wave to travel from the top of the range gate to a point in the tissue defined by a bottom of the range gate and back to the top of the range gate. The received echo signals can be sampled at virtually any rate in order to provide accurate Doppler analysis of the blood flowing in the area of tissue defined by the range gate.
In accordance with another aspect of the present invention, the amount of echo data that can be analyzed and hence the size of the range gate is increased by compensating for the various delay times associated with the position of the different transducer elements with respect to a focal point of a Doppler pulse. First, the time required for an ultrasonic sound wave to travel from each transducer element to a focal point of the Doppler pulse is calculated. This value is then converted to a phase delay of the Doppler pulse transmitted from each element. The phase of the Doppler pulse transmitted from each transducer element is then adjusted such that the phase-adjusted Doppler pulses will add constructively within an area of tissue defined by the range gate when simultaneously transmitted. By transmitting all the Doppler pulses simultaneously, the echo signals that originate within the range gate can be received for a longer period of time. The received echo signals can be sampled at virtually any rate to more accurately analyze moving blood flow.